在匿名类中使用方法参数会导致SIGSEGV

Question

I'm writing Android application that has Java and native part. Java part sends messages to native part and receives answers back. Native part does all work on separate thread and when it returns answer I want to handle answers on main thread. Here is part of my extended Application class:

@Override
public void OnMessage(final Message msg, final long answerTo) {
    Log.i(TAG, msg.ToStr()); // OK
    handler.post(new Runnable() {
        @Override
        public void run() {
            Log.i(TAG, msg.ToStr()); // Fatal signal 11 (SIGSEGV)
                                     // at 0x74616862 (code=1), thread 13255
        }
    });
}

Native code calls OnMessage method on it's thread and trying to pass it to UI thread through Handler. And when I try to use any method of msg inside UI thread my program fails with SEGSEGV.

Significant fact is that Message class is the wrapper for C++ Message class. Wrapper was generated by SWIG .

I tried to debug it in GDB, and GDB even shows me stack trace, which ends in native Message.toStr method. But gdb refuses to print variables, saying "No symbol " * " in current context".

Please, help me to resolve this issue.

Answer 1

I don't think you are using Handler properly for what you are trying to do (copy object across threads). Check out the blog post here:

http://techtej.blogspot.com/2011/02/android-passing-data-between-main.html

Specifically the sending of messages to handlers like this:

Message msg = Message.obtain();
msg.obj =  // Some Arbitrary object
mHandler.sendMessage(msg);

I don't think the way you are doing this performs the Handler magic of data copy between threads because it's just running a Runnable.

EDIT: I am curious to see if this was the issue, so even if it wasn't could you respond in a comment and let me know the outcome?

EDIT 2: So it looks like your object is probably being stored as a local ref in the JNI Layer. Unfortunately, that's not good enough for your purposes, you may need to make it a global ref. Just be wary of the fact that if you do make it a global ref, you have to delete it yourself in native code when you are done with it.

http://developer.android.com/training/articles/perf-jni.html#local_and_global_references

Answer 2

Finally I solved problem by myself. The problem was, that when we make call from C++ to Java, SWIG proxy method passed pointer to it's argument to Java side. Something like:

void SwigDirector_NativeLayerDelegate::OnMessage(Message msg, Long answer_to) {
  ...
  *((Message **)&jmsg) = &msg;
  ...
  jenv->CallStaticVoidMethod(..., jmsg, ...);
  ...
}

On Java side another proxy method recieved pointer, wrapped it with Java representation of Message class and passed it to Java method OnMessage:

  public static void SwigDirector_NativeLayerDelegate_OnMessage(
     NativeLayerDelegate self, long msg, long answer_to) {
    self.OnMessage(new Message(msg, false), answer_to);
    // false parameter means that Message object isn't owner of 'msg' pointer, so it
    // shouldn't free it on finalize.
  }

After OnMessage finished, native Message object was destructed in SwigDirector_NativeLayerDelegate::OnMessage and Java Message object kept pointer to destructed native object.

Solution

I've wrote custom typemaps for my Message object:

%typemap(directorin,descriptor="L$packagepath/$javaclassname;") Message
  %{*((Message**)&$input) = new Message($1);%}
%typemap(javadirectorin,descriptor="L$packagepath/$javaclassname;") Message
  %{new Message($1, true)%}

Now SwigDirector_NativeLayerDelegate::OnMessage creates copy of msg and Java object owns it:

// Native
void SwigDirector_NativeLayerDelegate::OnMessage(Message msg, Long answer_to) {
  ...
  *((Message**)&jmsg) = new Message(msg);
  ...
  jenv->CallStaticVoidMethod(..., jmsg, ...);
  ...
}

// Java
public static void SwigDirector_NativeLayerDelegate_OnMessage(
     NativeLayerDelegate self, long msg, long answer_to) {
    self.OnMessage(new Message(msg, true), answer_to);
}